/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* midNode(ListNode* head)
    {
        ListNode* fast = head;
        ListNode* slow = head;
        ListNode* pre = nullptr;
        while(fast && fast->next)
        {
            fast = fast->next->next;
            pre = slow;
            slow = slow->next;
        }
        pre->next = nullptr;
        return slow;
    }
    ListNode* mergeList(ListNode* l1, ListNode* l2)
    {
        ListNode dummy;
        ListNode* cur = &dummy;
        while(l1 && l2)
        {
            if(l1->val < l2->val)
            {
                cur->next = l1;
                l1 = l1->next;
            }
            else
            {
                cur->next = l2;
                l2 = l2->next;
            }
            cur = cur->next;
        }
        cur->next = l1 ? l1 : l2;
        return dummy.next;
    }

    ListNode* sortList(ListNode* head) {
        if(head == nullptr || head->next == nullptr)
            return head;
        ListNode* tmp = midNode(head);

        head = sortList(head);
        ListNode* l2 = sortList(tmp);
        return mergeList(head, l2);
    }
};
